Method and communications system for estimating an error covariance matrix for the downlink in cellular mobile radio telephone networks with adaptive antennae

ABSTRACT

Data is transmitted in a radio communication system having two or more base stations that are located in the network and having additional radio stations, which are each connected to one of the base stations via radio interfaces. At least one first base station has an antenna array with a multitude of antenna elements and with a signal processing device for the directional transmitting-receiving of data. The first base station temporally overlays data to a radio station, which is connected to the base station, for the transmission of data from an external base station to an external radio station connected thereto. The transmission of data from the first base station is also received from the external radio station. The objective is to reduce the amount of disturbance to the external radio station caused by transmissions from the base station in the downlink direction. To this end, the transmitting power of the antenna array of the base station is reduced in the direction toward the external radio station after a transmission of a training signal of the transmitted signal for the external radio station and after the reception of an assignable training signal of the external radio station.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is based on and hereby claims priority to GermanApplication No. 100 252 87.7 filed on May 22, 2000, the contents ofwhich are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] The invention relates to directionally dependent control of thepower for the downlink in a cellular radio communication network withadaptive antennas.

[0003] In radio communication systems, information (for example speech,image information or other data) is transmitted with the aid ofelectromagnetic waves via an air interface between transmitting andreceiving stations (base station and subscriber station, respectively).The emission of the electromagnetic waves is performed in this case withthe aid of carrier frequencies that are in the frequency band providedon the respective system. In the case of the GSM (Global System forMobile Communication), the carrier frequencies are at 900, 1800 and 1900MHz. Frequencies in the frequency band of approximately 2000 MHz areprovided for future mobile radio systems using CDMA or TD/CDMAtransmission methods via the radio interface, for example the UMTS(Universal Mobile Telecommunication System) or other 3rd generationsystems.

[0004] The data transmission takes place via frames in the case of theseradio communication systems. A division of a broadband frequency domaininto a plurality of time slots of equal time duration is provided in thecase of a TDMA component (TDMA: Time Division Multiple Access). The timeslots are used partially in the downlink DL (downlink from base stationto subscriber station), and partially in the uplink UL (uplink fromsubscriber station to base station). One or more switching points aresituated therebetween. The same is repeated for further carrierfrequencies. Information of a plurality of connections is transmitted inradio blocks within the time slots. Radio blocks for user datatransmission currently include sections with data in which trainingsequences or midambles known at the receiving end are embedded.

[0005] The switching points can be defined synchronously in all cells ofthe radio communication system. In this case, a time slot in the overallradio communication system is used exclusively in the uplink UL orexclusively in the downlink DL. Additional flexibility is achieved bydefining the switching points asynchronously. In this case, some cellsof the radio communication system use one time slot for UL and othersfor DL.

[0006] Because the distance between transmitter and receiver frequentlyfluctuates strongly during operation, it is desired to match thetransmit power over up to a plurality of orders of magnitude, in orderto keep the ratio of energy per bit/noise-power density or the ratio ofsignal/interferer or carrier power/ interference power in the limit ortarget range. On the one hand, the receive power must be at a minimumlevel that is required for the desired surface quality, but on the otherhand as little interference as possible is to be produced.

[0007] DE 198 03 188 discloses a method and a base station for, inparticular, TDMA/CDMA transmission methods (CDMA: Code Division MultipleAccess), in the case of which the signals transmitted from the basestation in the downlink are specifically amplified in the direction ofthe assigned subscriber station, and attenuated in the other directions.For this purpose, spatial covariance matrices are estimated in the basestation for each subscriber station in order to determine amplifyinginterference from the signal received in the uplink, and thereafter abeam-shaping vector is calculated which maximizes the signal/noise ratioat the receiver. A general eigenvalue problem is solved in this casewithout iteration. Thereupon, transmitted signals are weighted with thebeam shaping vector for the corresponding radio link and fed to theantenna element of the antenna arrangement for emission. The covariancematrix is determined from a priori assumptions with the aid of amathematical model. The base station measures nothing during thetransmission to its subscriber station in the corresponding downlinktime slot. Consequently, uplink measurements of the training sequencesmust be used for estimation, in order to estimate the downlinkcovariance matrix.

[0008] In other words, in the case of this method the antenna gain ofthe antenna arrangement of the base station is maximized in specificdirections, which are assigned to the dedicated subscriber stations, byappropriately driving the individual antenna elements of the antennaarrangement. That is to say, the power that is transmitted from anantenna group to an assigned subscriber station is emitted in amaximized fashion by constructive interference in the direction in whichthis subscriber station is located.

[0009] These radio communication systems have a cellular structure inwhich in each case a base station with at least one transmitting antennaarrangement supplies subscriber stations in a specific radio cell zone.In this case, disturbing interference with subscriber stations ofadjacent radio cell zones can arise that are supplied from a neighboringbase station. This is the case, in particular, when the base stationtransmits to a subscriber station assigned to it in a time slot in whichthe subscriber station in the adjacent radio cell zone receives datafrom its base station, the adjacent one.

SUMMARY OF THE INVENTION

[0010] An object of the invention is to provide a method for reducingthe downlink transmit power which disturbs subscriber stations inadjacent cells, and a cellular radio communication network with adaptiveantennas for carrying out such a method.

[0011] In the case of a method according to the present invention, theinterference at foreign subscribers is advantageously minimized bymaximizing the transmit power for dedicated subscribers of atransmitting or base station. In this case, the radio waves are directedin the direction of the desired dedicated subscriber stations and, inaddition, the transmit power in other directions is minimized.

[0012] The use of the method or of the radio communication system issuitable, in particular, for mobile radio networks that use a timedivision duplex (TDD) method with adaptive antenna groups. In the caseof the planned systems, these are, for example, UMTS UTRA-TDD andTD-SCDMA for China.

[0013] The use in FDD systems, for example GSM, is possible by means ofa frequency transformation that is carried out before the estimateduplink covariance matrices can be used for application as downlinks.

[0014] In the case of asynchronous switching points, it is advantageousalso to take account of the training signals of those foreignnetwork-side base stations that transmit downlink in an uplink time slotof the base station.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] These and other objects and advantages of the present inventionwill become more apparent and more readily appreciated from thefollowing description of the preferred embodiments, taken in conjunctionwith the accompanying drawings of which:

[0016]FIG. 1 is a block diagram of a mobile radio system,

[0017]FIG. 2 is the frame structure of a known TDD transmission method,and

[0018]FIG. 3 is a simplified block diagram of a base station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings, wherein like reference numerals refer to likeelements throughout.

[0020] The mobile radio system illustrated in FIG. 1 as an example of aradio communication system includes a multiplicity of mobile switchingcenters MSC that are networked to one another and/or provide access to afixed network PSTN or packet data network GPRS. Furthermore, thesemobile switching centers MSC are connected to in each case at least onedevice RNM for allocating radio resources. Each of these devices RNM inturn renders it possible to make a connection to at least one basestation BS, here the base stations BS and an adjacent base station BSn.Such a base station BS can set up via an air interface V a connection tosubscriber stations, for example mobile stations MS or other types ofmobile and stationary terminals. At least one radio cell Z is formed byeach base station BS. A plurality of radio cells Z can also be suppliedper base station BS in the case of sectorization or of hierarchic cellstructures.

[0021] Existing connections V1, V2 for transmitting user information andsignaling information between subscriber stations MS and a base stationBS, as well as a request for resource allocation or a shortacknowledgement message in an access channel RACH by a furthersubscriber station MS are illustrated in FIG. 1 by way of example. Theadjacent base station BSn is connected to a further subscriber stationthat is also denoted below from the point of view of the base station BSforeign to it as foreign or adjacent subscriber station MSn.

[0022] Also illustrated is an organization channel (BCCH: BroadcastControl CHannel) that is provided for transmitting user and signalinginformation with a defined transmit power from each of the base stationsBS for all subscriber stations MS.

[0023] An operation and maintenance center OMC implements monitoring andmaintenance functions for the mobile radio system or for parts thereof.The functionality of this structure can be transferred to other radiocommunication systems, in particular for subscriber access networks withwireless subscriber access.

[0024] The frame structure of the radio transmission may be seen fromFIG. 2. In accordance with a TDMA component (TDMA: Time DivisionMultiple Access), a division of a broadband frequency band, for examplethe bandwidth B=5 MHz, is divided into a plurality of time slots ts ofequal time duration, for example 16 time slots ts0 to ts15. A frequencyband extends over a frequency domain B. A portion of the time slots ts0to ts8 is used in the overall radio communication system in the downlinkDL, and a portion of the time slots ts9 to ts15 is used in the uplinkUL. Situated therebetween are one or more synchronous switching pointsSP—only one switching point in FIG. 2. In the case of this TDDtransmission method, the frequency band for the uplink UL corresponds tothe frequency band for the downlink DL. The same is repeated for furthercarrier frequencies.

[0025] Information of a plurality of connections is transmitted in radioblocks within the time slots ts. Function blocks for user datatransmission include sections with data d in which training sequences ormidambles ma1 to ma-n known at the receiving end are embedded. The datad with 1 . . . N symbols are spread individually by connection with afine structure, a subscriber code c, such that, for example, nconnections can be separated at the receiving end by means of these CDMAcomponents (CDMA: Code Division Multiple Access). A physical channel isformed in this case by a frequency band B, a time slot, for example ts6,and a subscriber code c. A plurality of physical resources are generallylinked to a logic channel in order to transmit services at high datarates. For example, 8 physical resources are required in each case forthe service 144 kbit/s in uplink and downlink.

[0026] The spreading of individual symbols of the data d has the effectthat Q chips of duration Tchip are transmitted within the symbolduration Tsym. The Q chips in this case form the connection-specificsubscriber code c. Furthermore, a guard period gp for compensatingdifferent travel times of the signals of the connections is providedwithin the time slot ts.

[0027] As may be seen from FIG. 3, the base station BS has atransmitting/receiving device TX/RX that subjects the transmittedsignals to be emitted to digital/analog conversion, transforms them fromthe baseband into the frequency domain B of the emission, and modulatesand amplifies the transmitted signals. The amplified signals are thenfed to the intelligent and/or adaptive antenna arrangement with theantenna elements A1-A4. A signal generating device SA has previouslyassembled the transmitted signals in radio blocks and assigned them tothe corresponding frequency channel TCH. A signal processing device DSPevaluates received signals received via the antenna arrangement and thetransmitting/receiving device TS/RX, and executes a channel estimation.

[0028] In order to reduce the interference of the base station BS, alsodenoted below as interfering base station BS, exerted on the adjacentand disturbed subscriber station MSn, an error covariance matrix isestimated in the disturbing base station BS. While the known covariancematrices serve the purpose of amplifying the transmitted signals in thedirection of the communicating subscriber stations MS, the errorcovariance matrix is formed in order to reduce the transmit power in thedirection of the adjacent, disturbed subscriber station(s) MSn.

[0029] Correlation signals are transferred to the disturbing basestation BS from the adjacent base station BSn, which is connected to orcommunicates with the subscriber station BSn assigned to it anddisturbed. In the case illustrated, the correlation signals aretransferred via the lines L1 and L2, which connect the two base stationsBS, BSn to the device RNM for administering radio resources.

[0030] Here, the disturbed subscriber station MSn transmits the trainingsequence(s) ma-n and/or the code of the training sequence(s) ma-n ascorrelation signals. The disturbing base station BS thereby detects thesignal of the foreign, disturbed subscriber station MSn, and cansimultaneously determine the intensity of this signal. Moreover, thedisturbing base station BS can use its antenna arrangement with theantenna elements A1-A4 to determine or estimate the direction from whichthis signal arrives, and thus the direction in which the disturbedsubscriber station MSn is located.

[0031] Using the received training sequence(s) ma-n, which are currentlyformed by a coded pilot signal, the disturbing base station BSn thencorrespondingly carries out a channel estimate for one or more foreignsubscriber stations MSn.

[0032] The result in the final analysis is the formation of an errorcovariance matrix R_(I) ^((k)) that is used to minimize the disturbingtransmitted signal to the disturbed subscriber station MSn. Thedetermination of the error covariance matrix R_(I) ^((k)) for thepurpose of reducing or minimizing the transmit power in the direction offoreign, disturbed subscriber stations MSn is performed in this case ina way comparable to the determination, known per se from DE 198 03 188A1, of the covariance matrix R_(S) ^((k)) for maximizing the transmitpower in the direction of dedicated subscriber stations MS. The sameholds for the determination of corresponding beam-shaping vectorsw^((k)), generalized eigenvalues λ^((k)) and the estimated uplinkchannel pulse response matrices H^((k)).

[0033] Finally, the ratio${r( w^{(k)} )} = \frac{w^{{(k)}H}R_{S}^{(k)}w^{(k)}}{w^{{(k)}H}R_{I}^{(k)}w^{(k)}}$

[0034] is maximized, the index k with 1≦k≦K and K as the number of thesubscriber stations MS to be taken into account. In this case, the beamshaping vectors w^((k)) are an M-dimensional vector with M (with M=4 inFIG. 3) as the number of the antenna elements A1-A4 of the antennaarrangement of the disturbing base station BS.

[0035] The quadratic Hermitian and positive-definite error covariancematrix R_(I) ^((k)), the number of whose rows and columns corresponds tothe number M of the antenna elements A1-A4, is formed from the sum ofthe total of L error covariance matrices R_(ad) ^((I)) for theindividual disturbed subscriber stations MSn of the adjacent radio cellsZn. It holds that:$R_{I} = {{\sum\limits_{l = 1}^{L}{R_{ad}^{(1)}\quad {with}\quad R_{ad}^{(1)}}} = {{\frac{1}{W} \cdot H^{(l)}}H^{{(1)}H}}}$

[0036] H^((I)) corresponding to the estimated uplink channel pulseresponse matrix of the I-th disturbed subscriber station MSn, and thesuperscript H marking the transjugation (“Hermitian operation”). Inorder to improve the accuracy of estimation, the estimates of thespatial error covariance matrix R_(I) ^((I)) can be undertaken by usinga rectangular or exponential window over a plurality of time slots thatmay stem from different frames. The subscriber-specific contribution canbe identified by correlation with the sets, transferred via thecommunication network, of training sequences ma-n. Consequently, thedisturbing base station BS can synthesize a predicted interference errorcovariance matrix R_(I) ^((I)) for the downlink DL for the subscriberstations MSn that are active in a downlink time slot DL-ts.

[0037] Each base station BSn can automatically transmit all the trainingsequences ma-n, which are newly allocated in their radio cell zone Z, tothe adjacent base stations BS. Alternatively, however, it is alsopossible to reduce the signaling outlay by specifically transmittingtraining sequences ma-n when a subscriber station MSn establishes thatit is receiving signals from a foreign base station.

[0038] The transmission of training sequences ma-n is preferablyperformed with the aid of a protocol that is, in particular, set upappropriately on the side of the network (RAN/Radio Access Network) ofthe radio communication system.

[0039] The protocol informs the adjacent, disturbing base stations BS atleast as to which subscriber stations MSn have been allocated whichuplink training sequences ma-n. If the disturbing base station BSreceives such a training sequence ma-n, which permits a uniqueidentification, the (disturbing) base station BS can therefore assignthis received signal to the (disturbed) foreign subscriber station MSn.The (disturbing) base station BS is therefore capable of initiating anestimate of each contribution to the error covariance matrix which stemsfrom the foreign subscriber station MSn.

[0040] In the case of a particularly preferred embodiment, the protocolinforms the disturbing base stations BS as to which subscriber stationsMSn have been allocated which uplink training sequences ma-n in whichuplink time slots UL-ts. In this embodiment, it is assumed that a fixedassignment between uplink time slots UL-ts and downlink time slots DL-tsexists in the radio communication system. This is a preferred embodimentfor symmetric services that exhibit equally large traffic loading inboth directions of connection, as is the case, for example, intransmitting speech.

[0041] If such a fixed assignment cannot be assumed between UL-ts andDL-ts, the protocol of the above-named advantageous embodiment isextended. The BS is now additionally informed as to in which downlinktime slots DL-ts the subscriber stations MSn are to receive signals ordata from their base station BSn.

[0042] The base stations BS, BSn thus administer an association tableMEM sketched in FIG. 3, that contains data relating to adjacentsubscriber stations MSn, their training sequences ma-n and, preferably,the downlink time slots DL-ts assigned to the latter. Of course, itfollows therefrom that the disturbing base station BS receives andadministers in the uplink not only the signals of the dedicatedsubscriber stations MS, but also the signals of the foreign disturbedsubscriber stations MSn. In the downlink, by contrast, the signals tothe dedicated subscriber stations MS are amplified, and the signals inthe direction of the foreign disturbed subscriber stations MSn areattenuated. The set of active subscriber stations MS, MSn to beadministered by a base station BS is therefore different in uplink anddownlink.

[0043] For the case in which the training sequences ma-n are allocatedcentrally on the network side, the training sequences can also betransmitted directly from the central allocation point, for example thedevice for allocating radio resources RNM, to the base station BSn,which sets up a communication link, and to possibly disturbing adjacentbase stations BS.

[0044] In the case of TDD systems, in which the transmission isperformed in the same frequency band in uplink and downlink, the spatialcovariance matrices can be determined directly in conjunction, inparticular, with time slot information that has been communicated. Bycontrast, in FDD systems (FDD: Frequency Division Duplex) a frequencytransformation has to be carried out before the estimated uplinkcovariance matrices can be used for the application in downlinks.

[0045] For the case of adjacent radio cells Zn having a width that isonly very small by comparison with the cell of the disturbing basestation BS, it is also possible for a subscriber station to be disturbedin a radio cell behind the directly adjacent radio cell Zn of the basestation BS. In such scenarios, not only information relating to thesubscriber stations MSn of the directly adjacent radio cells Zn isneeded, but also relating to the subscriber stations of the more remoteradio cells, which are therefore also treated like adjacent radio cellsZn.

[0046] The error covariance matrix R_(I) ^((I)) can advantageously alsotake account of and include the disturbing interference that is knownper se from DE 198 03 188.

[0047] Advantageous error covariance matrices follow from the a priorimodel for two- or three-dimensional isotropic noise in which it isassumed that mutually uncorrelated homogeneous plane waves of equalintensity are irradiated onto the BS from all directions. The associatederror covariance matrices can be specified in closed form and stored.

[0048] The invention has been described in detail with particularreference to preferred embodiments thereof and examples, but it will beunderstood that variations and modifications can be effected within thespirit and scope of the invention.

1. A method for data transmission in a radio communication system havingtwo or more network-side base stations (BS, BSn) and a multiplicity offurther radio stations (MS, MSn) that are connected in each case to oneof the base stations (BS, BSn) via air interfaces (V1, V2, Vn), at leasta first one of the base stations (BS) having an antenna arrangement witha multiplicity of antenna elements (A1-A4) and a signal processingdevice (DSP) for transmitting/receiving data as a function of direction,the first of the base stations (BS) transmitting data to a radio station(MS) connected to it in a fashion overlapping in time with thetransmission of data from a foreign base station (BSn) to at least oneforeign radio station (MSn) connected thereto, and the transmission ofdata from the first base station (BS) being noticed as disturbance forthe foreign radio station (MSn), characterized in that the transmitpower of the antenna arrangement of the base station (BS) in thedirection of the at least one foreign radio station (MSn) is reducedafter a transmission of an item of information via the transmittedsignal of the foreign radio station (MSn) and after the reception of anassignable signal of the foreign radio station (MSn).
 2. The method asclaimed in claim 1, in which transmitted to the base station (BS) asinformation relating to the transmitted signal of the at least oneforeign radio station (MSn) is/are the training signal(s) (ma-n) thereofand, optionally, the uplink time slot (UL-ts) thereof.
 3. The method asclaimed in claim 1 or 2, in which the information relating to thetransmitted signal of the foreign radio station (MSn) is transmitted vianetwork-side devices (L1, RNM, L2) in the radio communication system. 4.The method as claimed in a preceding claim, in which in order to set thedistribution of the spatial transmit power at the base station (BS)there is determined or estimated a spatial error covariance matrix R_(I)^((l)) which yields a beam-shaping vector (w^((k))) as solution to theoptimization problem${r( w^{(k)} )} = {\frac{w^{{(k)}H}R_{S}^{(k)}w^{(k)}}{w^{{(k)}H}R_{I}^{(k)}w^{(k)}} = {\max!}}$

and transmitted signals of the base station (BS) are weighted with thebeam-shaping vector (w^((k)).
 5. The method as claimed in claim 4, inwhich the solution to the optimization problem is performed by solvingthe general eigenvalue problem with positively semi-definitely Hermitianmatrix R_(S) ^((k)) and positively definite Hermitian matrix R_(I)^((k)), R _(S) ^((k)) w ^((k)) =λR _(I) ^((k)) W ^((k)) the beam-shapingvector w^((k)) being yielded, in particular, as eigenvector relating tothe maximum eigenvalue.
 6. The method as claimed in a preceding claim,in which the transmission of data in the FDD or TDD mode is performed intime slots and/or using the multi-slot method.
 7. The method as claimedin a preceding claim, in which the information relating to thetransmitted signal of the foreign radio station (MSn) is transmitted tothe base station (BS) regularly or after setting up a connection of theforeign base station (BSn) to the foreign radio station (MSn).
 8. Themethod as claimed in a preceding claim, in which transmitted in additionas information relating to the transmitted signal of the foreign radiostation (MSn) are the downlink time slots (DL-ts) in which the foreignradio station MSn receives data from its base station (BSm).
 9. Themethod as claimed in a preceding claim, in which, in addition, thedisturbing interference that does not originate from the radiocommunication system itself is also taken into account with the aid ofthe associated covariance matrix.
 10. The method as claimed in apreceding claim, in which information relating to training signals(ma-n) and uplink time slots (ts0-ts8) of the foreign network-side basestation (BSn) is transferred to the base station (BS) in a radiocommunication system with asynchronous switching points (SP) if thesetime slots are used by the base station (BS) in the downlink (DL). 11.Radio communication system, in particular for carrying out a method asclaimed in a preceding claim, having two or more network-side basestations (BS, BSn), a multiplicity of further radio stations (MS, MSn)that are connected in each case to one of the base stations (BS, BSn)via air interfaces (V1, V2, Vn), and at least one antenna arrangementwith a multiplicity of antenna elements (A1-A4) and a signal processingdevice (DSP) for transmitting/receiving data as a function of direction,in the case of at least a first one of the base stations (BS), the firstof the base stations (BS) transmitting data to a radio station (MS)connected to it in a fashion overlapping in time with the transmissionof data from a foreign base station (BSn) to a foreign radio station(MSn) connected thereto, and the transmission of data from the firstbase station (BS) also being received by the foreign radio station(MSn), characterized in that the signal processing device (DSP) isdesigned for reducing the transmit power of the antenna arrangement ofthe base station (BS) in the direction of the foreign radio station(MSn) after a transmission of an item of information via the transmittedsignal of the foreign radio station (MSn) and the reception of anassignable signal of the foreign radio station (MSn).
 12. The radiocommunication system as claimed in claim 11, in which an associationtable (MEM) buffers data (ma-n) of the foreign radio station (MSn)referring to downlink time slots DL-ts assigned to the latter, inparticular the information via the transmitted signal thereof, after thetransmission thereof, and provides them to the signal processing device(DSP).